Is it obligatory to perform ghusl for someone who experienced a wet dream, then forgot about it and remembered after a period of changing their clothes, and does not know whether anything was discharged or not?

One who has a wet dream in their sleep and does not see any semen upon waking is not required to perform ghusl. The person mentioned is not obligated to perform ghusl unless they are certain that semen has been discharged due to the wet dream. Ghusl is not required merely based on doubt about the presence of semen, for the default state is not janabah until it is confirmed with certainty.